The influencing factors of the volumetric efficiency of twin screw pumps are complex and the calculation of volumetric efficiency is difficult. To solve this problem, a volumetric efficiency modeling optimization method based on theoretical model and response surface method was proposed. The theoretical model of the twin screw pump efficiency was established based on the theory of parallel plate liquid flow, and the sensitivity analysis was carried out. The most sensitive design parameters affecting the pump volumetric efficiency were screened out, which included the clearance, pressure difference and dynamic viscosity. Based on the orthogonal experiment and response surface method, the fitting relationship model between pump volume efficiency and sensitive factors was established through the finite element numerical simulation analysis. The simulation results show that the selected sensitive parameters have significant effect on the efficiency. Based on the fitting relationship model, taking the volumetric efficiency as the optimization objective, the optimization model was established and solved to obtain the optimal combination of sensitive parameters, by which the volumetric efficiency of the pump was significantly improved. The proposed optimization method can reduce the complexity of the simulation and improve accuracy of the theoretical models.